Many electronic converters convert bi-polar signals from one form to another form using reference sources of both positive and negative polarity. For example, a dual slope, ratiometric type, analog-to-digital (A/D) converter converts bi-polar analog signals into digital signals by comparing the analog signal to be converted to a reference source of polarity opposite to the polarity of the analog signal and using the results of the comparison to produce the digital equivalent of the analog signal. The comparison is accomplished by using the magnitude of the analog signal to control the rate of charging of the capacitor of an integrator for a known period of time. Thereafter, the integrator charge is dissipated at a rate controlled by the magnitude of an accurate reference source. The period of time required to discharge the capacitor to a predetermined level, usually zero, is directly related to the magnitude of the analog signal. The time period is usually measured by enabling a counter to count clock pulses during the discharge or deintegrate period of time. The number of pulses counted is, thus, the digital equivalent of the magnitude of the analog signal.
Another example of an electronic converter requiring reference sources of both positive and negative polarity is a bi-polar digital-to-analog (D/A) converter. In such a converter a reference source is connected to charge a capacitor at a rate related to the magnitude of the reference source for a period of time related to the value of the digital signal to be converted. The polarity of the reference source, of course, is related to the sign (+ or -) of the digital signal.
In the past, bi-polar electronic converters have usually required the inclusion of two accurate reference sources, one of positive polarity and the other of negative polarity. Depending on the nature of the converter, the reference sources have been either voltage sources or current sources. Regardless of their nature both reference sources were required to be highly accurate. In the case of voltage sources this usually meant the inclusion of a potentiometer for each source and its adjustment to achieve full-scale readings. Such a technique is costly because it requires two potentiometers and the labor required to adjust both potentiometers. An alternative technique for achieving two highly accurate reference voltage sources is to use a potentiometer to establish one reference voltage and a precision voltage divider to establish the second reference voltage. While this technique is less costly than a technique utilizing two potentiometers, it still requires precision matched components, which are relatively expensive. A still further technique is commonly referred to as the "flying" capacitor technique. In this technique a capacitor is charged by a single voltage reference. Analog switches are used to connect either the positive or negative terminals of the capacitor to the input of the converter and the other terminal to ground, when the reference source is needed. This technique, of course, requires accurate component selection and a series of analog switches. As a result, it is still more complicated and expensive than desirable. The present invention is directed to eliminating the need for a second accurate reference source entirely and, thus, entirely avoiding these and other such problems.
Therefore, it is an object of this invention to provide a new and improved bi-polar converter with a single polarity accurate reference source.
It is another object of this invention to provide a bi-polar ratiometric type electronic conversion system including only a single polarity accurate reference source.
It is a still further object of this invention to provide a bi-polar analog-to-digital converter with a single polarity accurate reference source.
It is yet another object of this invention to provide a bi-polar digital-to-analog converter that includes a single polarity accurate reference voltage source.
It is still yet another object of this invention to provide an electronic converter that can be used as a bi-polar analog-to-digital converter or a bi-polar digital-to-analog converter yet only includes a single polarity accurate reference source.